The present invention relates to a method and apparatus for water treatment which can satisfactorily treat liquids and attain a heightened effect with respect to disinfection, etc.
Industrial and household wastes cause air pollution, water pollution in rivers, lakes, and marshes, etc., and there is concern about the influence of these pollutants on the environment and the human body. There is an urgent need to take technical measures to overcome this problem. For example, in water treatment for drinking water production and in sewage treatment and wastewater treatment, chemicals including chlorine have been used for decolorization, reduction in COD, and disinfection. However, there is a tendency toward prohibiting the use of chlorine because the addition of a large amount of this chemical yields new dangerous substances, e.g., environmental hormones (exogenous endocrine disruptors) and carcinogenic substances. Furthermore, the incineration of wastes is regarded as problematic with respect to safety because the incineration generates combustion gasesxe2x80x94which can contain carcinogenic substances (dioxins) depending on the combustion conditions and these substances influence the ecological system. Novel techniques for overcoming this problem are being investigated.
An electrolytic process has conventionally been used for wastewater treatment. In this process, clean electrical energy is utilized to conduct electrolysis while controlling chemical reactions on the electrode surfaces, whereby hydrogen, oxygen, ozone, hydrogen peroxide, etc., are generated to indirectly decompose a substance to be treated, or the substance is adsorbed onto one of the electrodes to directly decompose the same.
Oxidation reactions on the anode are known to yield oxidizing agents effective in water treatment (e.g., chlorine and ozone) and to further yield active substances such as OH radicals under some conditions. The resultant water is generally called active water, functional water, ionic water, bactericidal water, or the like (see Kyxc3x4sansei Denkaisui No Kisochishiki, Ohm-sha, Ltd.). Details of electrodes, substances to be reacted, etc., are given, e.g., in Denki Kagaku, Vol.62, 1084xe2x80x94(1992) and Journal of Applied Electrochemistry, Vol. 21, 104 (1991). These references point out that there are cases where a substance to be decomposed cannot be sufficiently decomposed depending on the electrode performance. In the electrolysis of an aqueous solution, the anodic oxidation reactions generally yield electrolysis products from the water. However, when an electrode catalyst which is highly reactive in electric discharge in water is used, there frequently are cases where the oxidation of other coexistent substances does not readily proceed. Examples of the material for electrodes where oxidation is conducted include lead oxide, tin oxide, platinum, DSA""s and carbon, while examples of the material for electrodes where reduction is conducted include lead, iron, platinum, titanium and carbon. Materials for use as electrode bases must have corrosion resistance from the standpoints of long life and avoiding the pollutants of the liquid to be treated. Consequently, useful materials for anode feeders are limited to valve metals such as titanium and alloys of the valve metals. Electrode catalysts also are limited to noble metals such as platinum and iridium and oxides thereof. It is, however, known that even when such expensive materials are used at a given electric current, they are gradually consumed and dissolved in the solution with the lapse of time at a rate corresponding to the current density. Hence, there is a need for an electrode having better corrosion resistance.
On the other hand, the usual cathodic reaction is the reduction of water, which generates hydrogen. Hydrogen has reducing ability and has use value in some applications. However, hydrogen not only is insufficient in its ability to decompose organic substances present in water to be treated, but is dangerous because it may explode when present in concentrations within a certain range. When oxygen is present, the reduction reaction of the oxidizing gas proceeds preferentially as a cathodic reaction to yield hydrogen peroxide. The effect of electrolytic treatment due to contact with a liquid to be treated is expected in addition to direct electrolysis as described hereinabove. For example, the function of superoxide anions (O2xe2x88x92), which are one-electron reduction products having high activity, is expected. Hydrogen peroxide is useful as a basic chemical indispensable to the food, medicine, pulp, textile and semiconductor industries. Attention is focused especially on future applications of hydrogen peroxide, such as the cleaning of electronic parts and the sterilization of medical tools, apparatus, and equipment. Although synthesized by the anthraquinone process, hydrogen peroxide itself is unstable and cannot be stored over long periods. Hence, there is a growing for an on-site hydrogen peroxide production apparatus from the standpoints of the danger of transportation and pollution abatement. An electrolytic process is most suitable for this application.
Journal of Applied Electrochemistry, Vol.25, 613xe2x80x94(1995) compares various electrolytic processes for yielding hydrogen peroxide. In these processes, it is necessary to supply an alkali ingredient as a feed material because hydrogen peroxide in each process is efficiently obtained in an aqueous alkali solution. Namely, an aqueous solution of an alkali such as KOH or NaOH is indispensable. In Journal of Electrochemical Society, Vol.140, 1632xe2x80x94(1993) reports the decomposition of formaldehyde with electrolytic hydrogen peroxide. Journal of Electrochemical Society, Vol.141, 1174xe2x80x94(1994) proposes a technique in which pure water as a feed material is electrolyzed using an ion-exchange membrane to synthesize ozone and hydrogen peroxide on the anode and cathode, respectively. However, this technique has a low current efficiency and is hence impractical. Although it has been reported that these techniques, when practiced under a high pressure, attain a heightened efficiency of synthesis, such processes are still impractical from the standpoint of safety. Furthermore, an electrolytic process using a palladium foil has been proposed, but applications thereof are limited because the attainable concentrations are low and the process is costly. When water containing a large amount of metal ions, such as, e.g., tap water, well water, or seawater is to be treated, there are cases where a hydroxide deposits on the cathode surface. For preventing such deposits, an apparatus for diminishing the metal ions by means of electrodialysis, a reverse osmosis membrane, or the like is indispensable as a pretreatment step, and it is necessary to periodically conduct cleaning or the like by feeding an acid. Namely, the management of chemicals is troublesome.
In the electrolysis of tap water, well water, or the like, the proportion of resistance loss to cell voltage is not negligible because the conductivity of the water is low. Furthermore, since the electrode area effective in reaction is limited, it is preferred to heighten the conductivity of the water and this has conventionally been accomplished by dissolving a salt such as sodium sulfate, potassium sulfate, sodium chloride, or potassium chloride. However, in the case of treating a liquid containing a large amount of metal ion, such as tap water, well water, or seawater, there is a fear that a hydroxide may deposit on the cathode surface to inhibit the reaction. Use of the apparatus employing, e.g., electrodialysis as a pretreatment step for preventing the deposition results in an increase in apparatus cost. Furthermore, the necessity of periodically feeding an acid for diminishing the deposit and cleaning the cathode surface results in troublesome management of chemicals.
As described above, a technique has conventionally been extensively employed in which ozone and hydrogen peroxide are electrochemically generated, and these chemicals are used to treat a raw liquid to disinfect the liquid or modify the same in various ways. However, this conventional technique has problems in that the bactericidal activity is insufficient. Also, under alkaline conditions, which are ordinary operating conditions, metal ions which may be contained in the liquid to be treated tend to be cathodically reduced and deposited as the hydroxides on the cathode surface.
It is therefore an object of the present invention to provide a method and apparatus for water treatment in which metal deposition is less apt to occur and in which a liquid can be treated at a higher efficiency than by conventional means.
The invention provides a method of water treatment which comprises providing an electrolytic cell having an anode chamber including an anode and a cathode chamber including a cathode, supplying an oxygen-containing gas to the cathode chamber and an acid-containing liquid to be treated comprising water to the anode chamber, passing an electric current through said electrolytic cell to generate hydrogen peroxide and an oxidation product by electrolysis, and contacting the liquid to be treated with said hydrogen peroxide and oxidation product to obtain treated water.
The invention further provides an apparatus for water treatment which comprises a first electrolytic cell for producing an acid and an alkali and a second electrolytic cell for producing hydrogen peroxide and an oxidant for treating a liquid comprising water said second electrolytic cell having an inlet for receiving the liquid to be treated and an outlet for receiving treated water, said apparatus further comprising a first pipe for supplying said acid produced by said first electrolytic cell into the liquid to be treated. In yet a preferred embodiment, the apparatus further comprises a second pipe for supplying alkali produced by said first electrolytic cell to the treated water.